Electronic display devices, such as mobile phones, laptop computers, PDAs etc, are usually designed to have a viewing angle as wide as possible so that they can be read from any viewing position, i.e., a public wide view mode. However, there are some situations where a display which is visible from only a narrow range of angles is useful, i.e., a private narrow view mode. For example, one might wish to read a private document using a portable computer while on a crowded train. A number of display devices are known that can be electronically switched to restrict the range of angles or positions from which the displayed information is comprehensible.
A method for switching from a public wide view mode to a private narrow view mode with no moving parts is to mount a light control film behind the display panel, and to place a diffuser which can be electronically switched on and off between the light control film and the panel. When the diffuser is inactive, the light control film restricts the range of viewing angles and the display is in private mode. When the diffuser is switched on, it causes light travelling at a wide range of angles to pass through the panel and the display is in public mode. It is also possible to mount the light control film in front of the panel and place the switchable diffuser in front of the light control film to achieve the same effect. Switchable privacy devices of these types are described in U.S. Pat. No. 5,831,698 (S. W. Depp; IBM 1998), U.S. Pat. No. 6,211,930 (W. Sautter; NCR Corp. 2001) and U.S. Pat. No. 5,877,829 (M. Okamoto; Sharp K. K. 2001). These technologies share the disadvantage that the light control film always absorbs a significant fraction of the light incident upon it, whether the display is in public or private mode. The display is therefore inefficient in its use of light. Since the diffuser spreads light through a wide range of angles in the public mode, these displays are also dimmer in public than in private mode. Another disadvantage relates to the power consumption of these devices. In the public mode of operation, the diffuser is switched on. This often means that voltage is applied to a switchable polymer-dispersed liquid crystal diffuser. More power is therefore consumed in the public mode than in the private mode. This is a disadvantage for displays which are used for most of the time in the public mode.
In GB2413394 (R. Winlow; Sharp 2005), a switchable privacy device is described that has one or more additional liquid crystal layers and polarisers added to the display panel. The intrinsic viewing angle dependence of these extra elements can be changed by switching the liquid crystal electrically in a well-known way. Devices utilising this technology include the commercially available Sharp Sh851i and Sh902i mobile phones.
WO06132384A1 (D. Kean; Sharp, 2006) discloses the use of an extra liquid crystal layer located between the existing polarisers of a liquid crystal display (LCD) panel. In this location, the extra switch cell can modify the greyscale curves for off axis light. This provides a higher level of privacy for images than the techniques disclosed in GB2413394.
GB2439961 (N. Smith; Sharp 2008) discloses the use of a switchable privacy device constructed by adding an extra cholesteric layer and circular polarisers to a display panel. The cholesteric layer can be switched between a public wide view mode and a private narrow view mode, providing 360° privacy for a given range of polar angles.
Methods that utilise an additional liquid crystal layer (GB2413394, WO06132384A1, GB2439961) add thickness to the resulting display module. Additional thickness is very undesirable, particularly in mobile display applications such as mobile phones and laptop computers.
Methods to control the viewing angle properties of an LCD by switching the single liquid crystal layer of the display between two different configurations, both of which are capable of displaying a high quality image to the on-axis viewer, are described in US20070040780A1 (P. Gass; Sharp 2007) and WO/2009/057417 (B. Broughton; Sharp 2009). These devices provide the switchable privacy function without the need for added display thickness, but require complex pixel electrode designs and other manufacturing modifications to a standard display.
One example of a display device that has privacy mode capability with no added display hardware complexity is the commercially available Sharp Sh7021S mobile phone. The privacy function in the SH702i is achieved via manipulation of the displayed image data in conjunction with the angular dependent data-luminance properties inherent to the liquid crystal mode. However, while in the private mode, the quality of the image displayed to the legitimate on-axis viewer is severely degraded.
A similar scheme to that used on the Sh7021S phone, but which manipulates the image data in a manner dependent on a second, masking, image, and therefore causes that masking image to be perceived by the off-axis viewer when the modified image is displayed, is described in GB2428152A1 (T. Wynne-Powell; Sharp 2007), US20070040780A1 and GB2457106A (B. Broughton; Sharp 2009). However, the image processing methods described in GB2428152A1 rely on the off-axis to on-axis luminance curve being strongly non-linear, which is not the case for some display modes, for example, in-plane switching (IPS) LC display modes and OLEDs. As a result, the privacy effect is not sufficiently strong in these display devices to disguise the on-axis image from an off-axis viewer.
It is therefore desirable to provide a high quality display with a switchable privacy feature whereby:                i) In the public mode, the on-axis and off-axis display performance (brightness, contrast resolution etc.) is not compromised.        ii) In the private mode, the on-axis display performance (brightness, contrast resolution etc.) has minimal degradation relative to the public mode.        iii) In the private mode, the privacy strength is sufficient to render images, text, etc. unintelligible to an off-axis viewer.        iv) No appreciable thickness or cost is added to the display module.        